The only time I have ever used reverb in a mastering setting was on an acoustic, live-to-2-track, semi-'classical' project: I needed to create a decent ring-out/decay for an otherwise good take that was spoiled by someone dropping something at the end. I programmed up a reverb that matched the sound of the room as best I could, and automated the send to it going up just on the last "boom" of the music, as the actual recorded tail was faded down before the offending thud could be heard. Worked nicely.

In any kind of pop or rock music, the thought of adding global reverb gives me the willies. No thanks. I just can't think of a situation where it would sound like anything other than cheese. ...

An ME did something similat for a recor I did a couple years ago. There was some kind of noise too close to the end of the song that made the fade seem abrupt. I can't even remember what song it was(I know what record), so that's a good sign.

I have had a few occasions when it made sense and I'm convinced it improved the song: a latin tune that was a tongue in cheek ballad about betrayal, a surf instrumental and an alt pop song that was mixed almost dry.

On occasion I've also put delay on an entire mix, mainly rockabilly stuff. That seems to work as well depending on the situation.

To reconstruct "chopped" fades which are unfortunately very common, mostly from not too experienced engineers I guess but every now and ten even from supposedly pro guys.
Turn up that volume knob to 11 or wear headphones when you fade out!
heh

Reverb in mastering is pretty uncommon, but you still need to have a good one available for the instances where it is called for. You may be reconstructing clipped tails or smoothing fades, occasionally the client wants a little extra on acoustic or folk music, sometimes you cheat on an orchestral or traditional/classical piece that wasn't recorded in a great hall, and finally, when doing a CD release of a film score a little 'verb may help you.